The diagnosis of deafness at an early age is most important to enable the early fitting of hearing aids and in the application of educational programs to assist language development in the hearing impaired child. Current procedures in the early diagnosis of deafness include the "Cribogram" and brain stem evoked response but neither has yet been developed to a stage where they can be implemented on a wide spread basis. At present, the earliest age for diagnosis of deafness is about seven to nine months, with the average being about 2 years of age.
Auditory evoked potentials recorded from the scalp in humans have now been described in many studies. These potentials have been classified into three main groups. These groups are:
(i) the brain stem evoked potentials which are approximately 0.5 microvolts in amplitude and occur within the first 10 milliseconds following an abrupt sound stumulus, usually a click;
(ii) middle latency responses which are approximately two microvolts in amplitude and occur between 7 and 50 msecs following the presentation of a click or tone pips and,
(iii) slow responses, about 10 microvolts in amplitude, following the onset of a tone burst and have latencies between 50 and 500 msecs.
Currently, the brain stem potential is receiving most attention both as a neurological and an audiological tool. It does, however, have the disadvantage of using abrupt stimuli. This is necessary since this response reflects synchronous firing patterns in the auditory pathway in the brain stem. Stimuli of slower onset fail to achieve the synchrony necessary for the recording of the various peaks. As a result of this limitation only high frequency hearing information is measured.